126 



plays little part in classification. Its structure is essentially as fol- 

 lows : There is an upper pseudo-cortical structure consisting of much 

 branched hyphal cells extending vertically, the walls of which are so 

 much gelatinized that it is impossible to observe the cell-outline ; 

 it is essentially a protective tissue though it also serves a mechanical 

 function ; below this is the algal layer, consisting of the algae 

 ( Cystococcus humicola] and a much-branched hyphal tissue, followed 

 by a rather reduced medullary tissue ; this is followed by a hyphal 

 tissue in which the filaments extend in the direction of growth ; there 

 is no lower cortical layer. 



The podetia or secondary thalli arise vertically from -the upper 

 surface of the primary thallus. It is the podetia which present the 

 great diversity of form and which bear the apothecia. They vary from 

 short and simple to much branched, nor is branching the only 

 structural change, for the simple or nearly simple, forms may vary 

 greatly in size and structure, some being nearly smooth, while others 

 bear numerous phyllodia or thalloid outgrowths structurally like the 

 primary thallus and which are functionally comparable to the leaves 

 of higher plants. 



In the majority of Cladoniae the primary thallus finally disap- 

 pears ; even the basal portion of the podetia dies away while there is 

 continual regeneration above. In some species the primary thallus 

 perhaps no longer exists or at least has only a very temporary dura- 

 tion. 



The podetia, no matter how variable their general conformation, 

 are typically radial in structure, built on the plan of a hollow cylinder ; 

 their structure, as revealed by transverse and longitudinal sections, 

 is as follows : There is an outer layer of tissue in all respects like 

 the upper layer of the horizontal thallus, and this is succeeded by the 

 algal layer in which the algae are quite deficient, which is followed 

 by the medullary tissue ; still more internally occur the longitudinal 

 hyphal bundles which form almost a complete cylinder. The hollow 

 central space is here and there traversed by hyphal bundles for the 

 purpose of preventing collapse ; mechanical adaptations are here 

 highlv developed. 



The apothecia are terminal upon the podetia, or upon short apothe- 

 cial stalks borne on the margin of the cup-shaped expansions of the 

 podetia. They vary in size from medium to large, and are either 

 simple or more or less agglutinate : the disk is convex ; the epithe- 



